1. Field of the Invention
The present invention relates to a flexible printed wiring board such as a TAB tape or a flexible printed circuit (FPC), which wiring board has wiring patterns and serves as a substrate for supporting semiconductor chips. More particularly, the invention relates to a wide flexible printed wiring board which has a large surface area per wiring pattern and which is used in a plasma display panel (PDP) or a similar device.
2. Description of the Related Art
Development of the electronics industry has been accompanied by sharp demand for printed wiring boards for mounting electronic devices thereon, such as IC (integrated circuit) chips and LSI (large-scale integrated circuit) chips. In the field of plasma display panels (PDPs), wider and more large printed wiring boards are used more than that of liquid crystal devices (LCDs) because the printed wiring boards for PDPs have wider wirings in wider pitch than those for LCDs in order to use e a higher voltage.
Such tape-form flexible printed wiring boards having a large width have drawbacks. For example, the flexible printed wiring boards are deformed during conveyance thereof, particularly uncoiling and coiling up thereof, thereby causing stress concentration in specific portions of the wiring boards. Therefore, after completion of a mounting step such as IC chip bonding, wire breakage in inner lead portions and cracking in solder resist occur.
More specifically, as shown in FIG. 4, after completion of production steps or product inspection, a flexible printed wiring board 1 is guided by means of guide rollers 2 and 3 and then coiled up by a reel 4. During the above conveyance, the flexible printed wiring board 1 is uncoiled to be conveyed in a horizontal direction and, via the guide roller 2, downward to the bottom of a U-shaped section 5, which acts as a buffer section. After passage through the bottom of the U-shaped section 5, the flexible printed wiring board 1 is conveyed upward to the guide roller 3. Since the direction of bending at the guide rollers 2 and 3 differs from that at the U-shaped section 5, stress tends to be concentrated in specific portions. In addition, as shown in FIG. 5, the flexible printed wiring board 1 is generally warped widthwise with the wiring-pattern-provided surface 1a inside. Also, the wider the width of the tape, the higher the degree of warpage. When such warpage is straightened at the guide rollers 2 and 3, stress is also concentrated in specific portions of the flexible printed wiring board 1.
In an attempt to solve the drawbacks of such flexible printed wiring boards, Japanese Patent No. 3350352 (Claims, paragraphs such as [0005]) proposes a carrier tape exhibiting enhanced resistance to stress. In the flexible printed wiring board, wirings provided on the film substrate have almost the same total length of non-parallel sections so as to attain uniform stress resistance in both the center and the periphery of the film.
However, the aforementioned drawbacks in relation to stress concentration are considered to be attributed to marginal portions where no wiring pattern is provided.
In an attempt to prevent generation of cracks and wire breakage in wiring patterns caused by repeated temperature changes effected during device mounting, Japanese Patent Application Laid-Open (kokai) No. 11-45913 (Claims, paragraphs such as [0010] and [0030]) proposes a flexible printed wiring board having a dummy pattern around a device hole, the dummy pattern being electrically unconnected with the wiring pattern. The dummy pattern assumes a shape similar to that of the wiring pattern and is provided such that substantial uniformity in pattern density and thermal expansion coefficient over the film carrier is attained.
However, even when such a dummy pattern is provided, the aforementioned problems in the production of flexible printed wiring boards are not completely solved.